A step-ratio or multiple-ratio automatic transmission in a vehicle powertrain utilizes various friction elements for automatic gear ratio shifting. The friction elements establish power flow paths from a torque source such as an internal combustion engine and/or a traction motor to vehicle traction wheels. The overall transmission speed ratio, which is the ratio of a transmission input shaft speed to a transmission output shaft speed, is reduced as the transmission upshifts through the various available gear ratios.
In the case of a synchronous upshift, an off-going clutch (OGC) is released while an on-coming clutch (OCC) is engaged to lower a transmission gear ratio and change the torque flow path through the transmission. A typical upshift event is divided into a preparatory phase, a torque phase, and an inertia phase. During the preparatory phase, the OCC is stroked to prepare for its engagement while the OGC torque-holding capacity is reduced as a step toward its release. During the torque phase, which may also be referred to as a torque transfer phase, the OGC torque is reduced toward a value of zero or an insignificant level to prepare for disengagement. Simultaneously, the OCC torque is raised from an insignificant level to initiate engagement of the OCC according to a conventional upshift control strategy. The timing of the OCC engagement and the OGC disengagement results in a momentary activation of two torque flow paths through the gearing, which may cause torque delivery to drop momentarily at the transmission output shaft. This condition, which can be referred to as a “torque hole,” may occur before disengagement of the OGC. A vehicle occupant may perceive a “torque hole” as an undesirable shift shock. When the OCC develops enough torque, the OGC is released, marking the end of the torque phase and the beginning of the inertia phase. During the inertia phase, the OCC torque is adjusted to reduce its slip speed toward zero. When the OCC slip speed reaches zero, the shift event is completed.
Torque hole filling is a control strategy that attempts to reduce and/or eliminate the transmission output torque hole during an upshift event. Control strategies for reducing torque disturbances include providing an increase in transmission input torque during the torque phase of the upshift. The increase in transmission input torque must be synchronized with the OCC and OGC to deliver a consistent shift feel. Various techniques and/or strategies may be used to increase transmission input torque from the engine or the electric motor. While electric motor torque can be increased nearly instantaneously when torque is available, engine torque is generally slower to respond due to the system dynamics of fuel and airflow. An engine torque increase may be initiated in anticipation of a transmission upshift by opening a throttle and providing additional fuel. The throttle may be opened more than required to achieve driver demand torque with spark retard used to maintain the desired torque so subsequent advancing of spark timing may be used to provide a faster torque increase during torque hole filling. This strategy creates a torque reserve where the engine can quickly provide more transmission input torque. However, there are various limitations associated with use of this approach; for example, external conditions (e.g., high altitude) may prevent the engine from creating the desired torque reserve, which would reduce the overall effectiveness of the torque hole filling strategy. Similarly, increased fueling of the engine with spark retard to provide a torque reserve may adversely impact fuel economy and feedgas emissions.